Embryology

I

Introduction

Embryology, branch of biology dealing with the development of the animal embryo. (For the embryology of plants, see Fertilization; Plant; Seed.) Embryology includes within its province the development of the fertilized egg and embryo and the growth of the fetus.

II

History

Until the second half of the 18th century, embryology was a matter of speculation rather than of knowledge. One generally accepted theory was that of preformation: The complete animal with all its organs was believed to exist in the germ in miniature, needing only to unfold like a flower. It followed that each germ must contain within itself the germs of all its future descendants, one within another, as in a nest of boxes. Many naturalists believed the germ to be contained in the ovum, the female germ cell, but after the microscope had revealed spermatozoa, the male germ cells, in 1677, a school of so-called spermists advanced the hypothesis that the germ was contained in the spermatozoon. Their drawings show the spermatozoon encasing a minute human figure, called the homunculus.

Little attention was given to the theory, called the theory of epigenesis, that the English physician and anatomist William Harvey had stated in 1651. This theory, which had been vaguely expressed much earlier by Aristotle, held that the specialized structures of the individual develop step by step from unspecialized antecedents in the egg. Proof of this theory was not forthcoming, however, until 1759 when the German anatomist Kaspar Friedrich Wolff reported on his study of the development of the chick in the egg and showed that the organs arise from undifferentiated material. The basic potential nature and organization of the structures of the organism are determined by the genetic constitution of the fertilized egg (see Heredity). Wolff is called the founder of modern embryology, a title also sometimes given to the Estonian naturalist Karl Ernst von Baer, who in the 19th century described the principal phases in the development of the chick and pioneered in comparative embryology.

A firm basis for the new science was established by the cell theory formulated in 1838 by the German botanist Matthias Jakob Schleiden, who stated that all plants and animals are made up of cells. A year later his compatriot, the anatomist and physiologist Theodor Schwann, confirmed this theory. In later work these men demonstrated that tissues and organs develop by cell division. See Cell.

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III

Normal Development in Animals

Development consists of a series of events beginning with fertilization of the egg. For a description of male and female germ cells, called gametes, and the process of nuclear fusion of these germ cells to produce a zygote, see Egg; Fertilization.

After fertilization, the egg undergoes cell division, or cleavage. Thus, one cell divides into two; the daughter cells, called blastomeres, then cleave into four; these cleave into eight, and so on. When the embryo consists of a hundred or more cells it may form a solid mass, called a morula from its resemblance to a mulberry. In most species the mass then resolves itself into a single layer of cells forming a hollow sphere, the blastula. The next step is the formation of a double-walled sac or cup, the gastrula. The outer wall is called the ectoderm, and the inner wall is the endoderm. The endoderm surrounds a new cavity known as the primitive gut. In some cases these two layers are formed by delamination, or splitting, of a mass of cells, but more commonly they are formed by invagination, that is, the pushing in of a portion of the wall of the blastula. In all animals except the simplest, a third layer, the mesoderm, develops between the other two layers.

These three layers, known as the primary germ layers, differentiate into analogous organs in all species of animals. The endoderm produces specialized cells in the principal digestive glands and forms the lining of the air passages and of most of the alimentary canal. The mesoderm gives rise to the blood and blood vessels, the connective tissues, the muscles, and usually the reproductive glands and the kidneys. The ectoderm gives rise to the epidermis and derivative structures such as the hair and nails, to the mucous membranes lining the mouth and anus, to the enamel of the teeth, and to the central nervous system.

IV

Embryonic Induction

One of the outstanding achievements in embryology in the 20th century has been the elucidation of some of the reasons for morphogenesis, that is, the development of pattern and form, and for differentiation, that is, the development of a diversity of cell and tissue types. Observation and experiment, especially on amphibian embryos, have shown that a stimulus emanates from some of the material that invaginates during the process of gastrulation. Those cells that invaginate on what will be the future dorsal side of the embryo have the capacity to induce overlying cells to differentiate into the primary axial organs and associated structures, such as the nervous system, notochord, and muscle segments.

If the potentially inducing cells making up the so-called dorsal lip of the blastopore, which is the opening of the cavity of the gastrula, are prevented from invaginating, the embryo remains alive but will not undergo further differentiation. Conversely, grafting a second dorsal lip to the flank of an embryo induces the formation of a secondary embryo out of tissues that normally would have formed something altogether different. Studies show that various chemical substances can imitate in part the stimulus or stimuli that derive from the inducing embryonic tissue.